This invention relates to a torque limiting clutch of the type comprising opposed first and second members mounted for relative rotational and axial movement, a third member positioned between and mounted for rotational movement relative to the first and second members and for axial movement relative to at least one of the first and second members, the third member being provided with a torque transmitting element receiving aperture, a torque transmitting rolling element being positioned in the aperture in the third member, the first member comprising a driven member and being provided with a torque transmitting abutment for torque transmitting engagement with said torque transmitting element, another of said members comprising a driving member and being provided with a torque transmitting abutment for torque transmitting engagement with said torque transmitting element, resilient loading means to bias resiliently the first and second members towards one another and the arrangement being such that when the clutch is transmitting torque the torque transmitting element is engaged with a driving member abutment and a driven member abutment so as thereby to prevent relative rotation between the driving and driven members and so that, on the torque to be transmitted exceeding a predetermined value, the torque transmitting element disengages from the abutment of at least one of the driving and driven members to permit of the members rotating relative to one another. Such a torque limiting clutch will be referred to hereinafter as "of the type described. "
A torque limiting clutch of the type described may be constructed so that the said first member comprises the driven member and said second member comprises the driving member. In this case the third member comprises only a torque transmitting element guide member and does itself not transmit torque.
Alternatively an overload clutch of the type described may be constructed so that said first member comprises the driven member and said third member comprises the driving member. In this case the third member acts both as a torque transmitting element guide member and also as the driving member, the aperture in the third member serving both as a guide means and as a torque transmitting abutment.
When a clutch of the type described is subjected to a torque in excess of the predetermined value for a long period of time performance of the clutch deteriorates due to repeated engagement and disengagement of the torque transmitting elements with the abutments.
There has been proposed in U.S. Pat. Nos. 3,722,644 and 3,774,738 torque limiting clutches of the type described provided with spacing key assemblies arranged to be movable from an inoperative position, occupied during normal torque transmission by the clutch, to an operative position, occupied when the clutch is disengaged, in which the keys are positioned between the clutch members to maintain the clutch members spaced apart, the torque transmitting elements are received in a receiving means in one of the members and spring means are provided to continuously urge the torque transmitting elements out of the receiving means against the other clutch member for engagement with torque transmitting abutments thereon in order to permit resetting of the torque limiter on relative rotation of the clutch elements in the reverse direction.
In practice after a torque limiting clutch of the type described has disengaged it frequently occurs that the driving member is continued to be rotated for a considerable period of time. Thus, in the case of the clutches disclosed in said U.S. patent specification on such continued relative rotation between the clutch members because of the continuous spring bias urging the torque transmitting elements out of their receiving means against the other clutch member the torque transmitting elements are impacted into, and out of, the torque transmitting abutments of the other member each time such an abutment passes each torque transmitting element. This leads to considerable wear of the torque transmitting elements, guide member and abutments and can lead to their eventual disintegration. Moreover the clutch may re-engage leading to damage of the apparatus which the clutch is intended to protect.